Body Composition
Hydrostatic Weighing
What is Body Composition?
• Body composition makes reference to a
number of things including
– Total amount of lean mass (very closely related
to muscle mass)
– Total amount of fat mass
– Percent lean mass – lean mass/total body mass
– Percent body fat – fat mass/total body mass
Two-Component Model
Human Body Composition
= Fat Mass + Fat-Free (Lean) Mass
i.e. Siri Formula
Density of FM = 0.9 g/mL
Density of FFM = 1.1 g/mL
Fat and Lean (Fat-Free) Mass
• Lean Mass (FFM): All of the body’s non-fat
tissues
– Bone, muscle, organs, and connective tissue
• Fat Mass
– Essential fat
• Men (3%), Women (12%)
– Storage fat: Found in adipose tissues
• Subcutaneous or visceral
• Functions: Insulator, substrate, padding
Body Composition is Calculated
From Body Density
• A relatively small number of studies have
been done in which cadavers are
dissected and the lean mass and fat mass
are separated, weighed, and their density
is determined
• From these studies regression equations
were developed to determine body
composition from body density
Why Are There 3 Different Formulas?
Each formula has been invented using
different “population.”
Therefore, each formula uses different
assumptions (Density of FFM vs FM).
Which formula should you use?
Ways to Determine Body Density
• Direct Measurements
– Hydrostatic Weighing
– Plethysmography (Bod Pod)
• Indirect Measurements
– Standard Tables
– Body Mass Index (BMI): Wt (kg)/Ht2 (m2)
– Skinfolds
– Bioelectrical Impedance Analysis (BIA)
Hydrostatic Weighing
• A direct measurement of body density
• Considered the gold standard of body
density measurement
Density…
Density = Mass / Volume
In order to obtain Density,
you have to measure Mass and Volume.
Mass: easy to measure…get on a scale.
Volume: get in the water!
(volume of water displaced = volume of your body)
A Little Physics…
Archimedes’ principal:
Buoyancy Force (Fb)
= Weight of Fluid Displaced
Body Density = Body Mass
Body Volume
= Volume of Water
Displaced
Underwater Weight – Dry Weight
Buoyancy Force
Acceleration Due to Gravity
=
Mass of Water Displaced
Density of Water
A Complication: Air in Your Body
AIR
↓
↑ Volume
&
↑ Buoyant Force
Where is it?
Air in Your Body
Lungs & Gastrointestinal Tract
How do you control them?
Air in Lungs:
test at RV
Air in GI Tract: minimize it by fasting
Problems
• Is RV accurate?
• Some people do not feel comfortable
submerging in the water at RV.
• GI gas is not measurable.
(could be anywhere from 0.1 to 1.5 L)
• Densities of FM & FFM are not always as
presented earlier.
(i.e. Osteoporosis patient?)
For Your Interest…
Dry Weight = mb*g
Fb = VH2O*DH2O*g = (Vb + Vair)*DH2O*g
Under Water Weight = Dry Weight – Fb
= mb*g – (Vb + Vair)*DH2O*g
since g is constant throughout this derivation, let’s
ignore it.
UWW (in kg) = DryW (in kg) – (Vb + Vair)*DH2O
Vb = DryW / Db
UWW = DryW – [ (DryW / Db) + Vair) ]*DH2O
UWW = DryW – (DryW / Db)*DH2O – (Vair)*DH2O
(DryW / Db)*DH2O = DryW – UWW – (Vair)*DH2O
Db = (DryW)*DH2O / [ (DryW – UWW) – (Vair)*DH2O ]
Db =
DryW
[ (DryW – UWW) / DH2O ] – (Vair)